When stripping road surfaces by milling and when removing mineral deposits with surface miners likewise by milling, the tools used, in particular milling chisels, are subject to a continuous wear process. When the tools reach a certain wear state, it is advisable to replace them or the efficiency of the remainder of the process decreases. In this connection, it is necessary to differentiate among different wear states that lead to the replacement of a milling chisel or chisel holder.
Replacement of the chisel may be needed because there is no longer sufficient wear material, particularly hard metal at the tip. The penetration resistance becomes too great, causing the efficiency to decrease, such as from excessive frictional loss. The wear is for the most part rotationally symmetrical.
Replacement of the chisel holder may be needed because the wear limit is reached and the holder begins to wear at the contact surface between the chisel and the holder. This wear is usually symmetrical.
Non-rotationally symmetrical wear may appear on the chisel tip and/or on the chisel head due to insufficient rotational motion of the chisel during the milling process. This results in a poor milling pattern as well as the risk of tool breakage since the supporting action of the chisel head is lost.
The chisel holder can also be subject to additional non-rotationally symmetrical wear.
The chisel may also need replacement because of breakage.
Furthermore, worn and/or broken chisels can lead to secondary damage to the chisel holders and worn chisel holders can lead to secondary damage to the milling roller. A prompt replacement of the chisels and/or chisel holders is thus necessary and reduces costs. By contrast, it is not optimal from a cost standpoint for the chisels and/or chisel holders to be replaced too early because the chisels and chisel holders, as wearing parts, are very expensive. This results in failure to properly utilize remaining wear potential. Up to now, the wear state of chisels and chisel holders is ascertained through visual inspection by the machine driver. In order to do so, the machine driver must shut down the machine or switch off the engine and decouple the roller from the drive train. The driver must then open the rear roller flap in order to visually inspect the milling roller.
The milling roller is then rotated by a second drive unit in order to permit inspection of the entire milling roller. The task of roller inspection can also be performed by a second operator. In this case, the wear state of the chisel holders is usually determined by known wear markings. The wear state of the chisels is determined by longitudinal wear and the rotational symmetry of the wear pattern.
Inspecting the wear state of the chisels and holders is very time-consuming and is also unproductive because no production can take place during this time. The entire process is interrupted, thus additionally reducing availability. Furthermore, due to the fact that the assessment is very prone to subjectivity, there is the risk of failure to optimally utilize the wear potential of the holders and chisels.
German Patent Reference DE 102 03 732 A1 discloses an apparatus in which operation can be optimized by monitoring operating states of machine components that are directly or indirectly involved in the milling process. Thus among other things, the wear state of the chisels is assessed by evaluating various machine parameters and characteristic values. During operation of the milling machine, one problem arises if the milling process and the subsurface itself, whose properties fluctuate widely, have a large influence on the evaluation of the operating state of components. Because of the widely fluctuating properties of the subsurface to be milled, it is not easy to differentiate between changing operating states of machine components directly or indirectly involved in the milling process that are attributable to changing surface properties and those that are attributable to changing tool properties.
Austrian Patent Reference AT 382 683 B discloses a mining machine in which the cutting roller is monitored in a contactless fashion. In this case, photoelectric detectors are used, which detect the presence of the chisels. This method is unable to provide a quantitative wear evaluation.